Austenitic heat resisting steel

ABSTRACT

Cr-Ni type austenitic heat resisting steel having about twice the heat resistibility of common stainless steel is provided by the addition of 0.001 to 0.30 percent by weight Ti, 0.001 to 0.30 percent by weight Nb + Ta, and 0.0001 to 0.05 percent by weight B to a well-known Cr-Ni type steel composition for elevated temperature service, and maintaining a specified balance of (Ti + 0.5 (Nb+Ta)) to C and a specified balance of Ti to (Ti+0.5 (Nb+Ta)).

United States Patent 11 1 Mimino et al. 1 Aug. 7, 1973 [54] AUSTENITIC HEAT RESISTING STEEL 2,562,854 7/l95l Binder 75/128 F 60 8 9 2 b' 7 128 F [76] Inventors: Tohru Mimino; Kazuhisa Kinoshita; Z1 2 3; n 9 F Minegighii Takayuki Shinodfl 2,801,9l6 8 1957 Harris... 75 128 T a of Guutsa K y 3,303,023 2/1967 Dulis 75/128 F Nippon Kokan Kabushiki Kaisha, 2730 Minamiwatarida cho, Kawasaki Japan Primary ExaminerHyland Bizot Att0rneyFlynn & Frishauf [22] Flledz Dec. 14, 1970 21 Appl. No.: 97,854

Related us. Application Data [57] ABSTRACT l conllnumion'in'pmt of 773786 61 Cr-Ni type austenitic heat resisting steel having about abandoned twice the heat resistibility of common stainless steel is provided by the addition of 0.001 to 0.30 percent by 52 U.S. c1. 75/128 G, 75/128 T weight Ti, 00m to 030 percent by weight Nb Ta, If-lt. Cl. and tG percent weight B to a held of Search T, F, known Cr Ni yp Steel Composition for elevated 75/128 G perature service, and maintaining a specified balance of [Ti 0.5 (Nb+Ta)] to C and a specified balance of [56] References Clled Ti to 5 UNITED STATES PATENTS 2,190,486 2/1940 Schafmeister 75/128 G 5 Claims, 2 Drawing Figures SUS 27 x k J NO. I T T l i No 2 STEELE ACCORD N6 3 l v TO 77-115 N 0 \7oc c 350C 550% 600 c some No 3 \1000 700 go IOOOhr moc 0hr IO 90hr l \IOOOhr: 4 j l /"L' 0 IO 20 3O Kg/mm- PAIENIEW v I 3.751.244

SEE! 1 0P2 SUS 27 NO. I

CREEP RlPTLRE STRENGTH Kg/mm AUSTENITIC HEAT RESISTING STEEL RELATED APPLICATION This application is continuation-in-part of application Ser. No. 773,786, filed November 6, 1968 now abandoned.

FIELD OF THE INVENTION This invention relates to Cr-Ni type austenitic stainless steels of improved heat resistibility for elevated temperature service.

BACKGROUND OF THE INVENTION It is well-known that higher strength and oxidation resistibility of steel at high temperatures are required in some fields, e.g. the boiler industry. As boilers become larger in size and of an ultra-critical pressure type, steels of higher strength have been called for by said service.

At present, JIS (Japanese Industrial Standard) SUS- 27, SUS-29, SUS-32 steels and the like (A1S1-304, 321 and 316 steels as similar standards) among 18 Cr 8 Ni austenitic stainless steels are generally employed for elevated temperature and high pressure services. The fact is that said SUS-32 steel (AISI-316 steel) among the above steels is commonly employed in view of its having superior strength at high temperatures in spite of a high cost which is due to a Mo content of 2.0 to 3.0 percent. The cost of SUS-27 steel (AISI-304 steel) is relatively low, while its high temperature strength is inferior to other stainless steels. SUS-29 steel (A181- 321 steel) formed by adding Ti to said SUS-27 steels, has higher heat resistibility than said SUS-27 steel. It is, however, found that the strength of said SUS-29 steel fails remarkably in service for a relatively long period of time such as 650C at 100,000 hours. Consequently, the strength is substantially the same as that of said SUS-27 steel.

Thus, an inexpensive heat resisting steel having higher strength has not yet been provided for industrial circles. Accordingly, the development of more economical heat resisting steels is required today.

The present invention has been developed in order to meet the above requirement. The heat resisting steels of this invention are characterized by the addition of 0.001 to 030 percent by weight 113E001 misses; cent by weight Nb+Ta and 0.0001 to 0.050 percent by weight B to compositions of the above Standards steels, and by critical values of Ti 0.5(Nb l Ta)/C and Ti/Ti 0.5(Nb+Ta).

SUMMARY OF THE INVENTION In accordance with the present invention, there are provided steels of the following composition in weight percent:

C 0.03 to 0.30 Si up to 1.00 Mn up to 2.00 Cr 15.0 to 26 Ni 7.0 to 22 Ti 0.001 to 0.30 Nb Ta 0.001 to 0.30 B 0.0001 to 0.050 'Fe and unavoidable balance impurities and having a ratio of Ti 0.5(Nb+Ta)/C of from about BRIEF DESCRIPTION OF THE DRAWINGS Additional objects of this invention will become apparent from the following description and the examples and the accompanying drawings in which:

Flg. l is a graph showing the relation of the creep rupture strength in the case where steels subjected to elevated temperature service are JIS SUS-27 (A181- 304), steels No. 1, No.2, No. 3 and No.4 according to this invention and No. 5 not containing B.

FIG. 2 is a graph illustrating the critical relationships Ti 0.5(Nb+Ta)/C and Ti/Ti 0.5(Nb+Ta), with respect to creep rupture strength.

SPECIFIC EMBODIMENTS OF THE INVENTION As mentioned above, the Ti and Nb Ta are effective for causing the agglomeration of produced carbides to disperse uniformly and finely by interaction of said additional elements.

The C content of the steels is chosen to be a relatively high value in order to cause Cr-carbides to precipitate and disperse around Ti and Nb-carbide and improve high temperature characteristics. However, as Crcarbide has a tendency to agglomerate around said Ti and Nb-carbide, the addition of B is effective to stop said agglomeration and cause the higher strength to hold.

The content of Ti should be defined to be within the above-mentioned range. More than 0.30 percent Ti causes its carbide to coarsen and causes its strength to decrease at high temperatures, while less than 0.001 percent Ti brings about a decrease of the amount of Ticarbide precipitated and is ineffective for improving its strength.

The effect of Nb Ta, too, is similar to that of Ti. That is, both more than 0.30 percent Nb Ta and less than 0.001 percent Nb Ta are unable to cause high temperature strength to improve. It is to be understood that Nb or Ta can be used in place of mixtures of Nb and Ta; however, generally mixtures are employed.

Less than 0.0001 B is ineffective for improving its strength at high temperatures, while more than 0.050 percent B isundesirable because workability of said steel decreases exceedingly, even though the high temperature strength may possibly be improved.

' Referring now to FIG. 1, each graph is plotted on the basis of the following examples.

The chemical composition (by weight percent) of the above examples is shown in Table 1.

TABLE I The composition of SUS-27 steel (AISI-304 steel) is based on the Standard.

TABLE 1 NoTEs.-1. A11 compositions No. 1, No. 2, No. 3, and No. 4 steel are within the ranges given above for steels of this invention. 2. The composition 01' N o. 5 steel is not within the ranges given above in View of the omission of B.

SUB-27 steel and steels Nos. 1-5 were manufactured by using a conventional process, and were then subjected to a creep rupture test at 600C, 650C and 700C in hours and 10 hours, respectively. The results are shown in the following TABLE 2.

As mentioned above the only difference between the compositions of No. l to No. 4 steels and No. 5 steel was the presence of B. Referring now to the above Table 2 and P16. 1, it is clearly indicated that there is a great difference in creep rupture test values for the steels tested. That is, it should be understood that the existence of B has an important effect upon the creep rupture strength. Even such steel as No.5 steel attained a higher strength as compared with said SUS-27 steel, by reason of the addition of Ti and Nb Ta to said SUS-27 steel. This fact evidently explains that addition strength at 650C. for 100,000 hours was determined by extrapolation. The creep rupture values, at 650C. for 100,000 hours, are given below in TABLE 4 and are also shown in FIG. 2.

TABLE-1 No 11 12 13 14 15 16 17 1s 10 Kg./mm.- 5.3 8.0 8.5 9.2 10.0100 0.1 7.5 7.5

As shown, steels 12 through 17 with ratios within a range of about 0.25 to 3, have substantially greater creep rupture values than steels l1, l8 and 19. In particular, it is to be noted that steels l8 and 19 contain relatively large amounts of Ti and Nb Ta, and yet are inferior to steels 12 through 17 which contain substantially lesser quantities of such components. This is unexpected.

The ratio Ti/Ti+0.5(Nb+Ta) is also controlled in order to achieve high creep rupture values. This ratio is maintained between about 0.15 and about 0.85, and preferably 0.l5+0.75. Criticality is shown by data with steels Nos. 21 through 28. Here again as with Nos. ll-l9, such steels do not contain 8. The components of Nos. 21-28 are given (in weight percent) in TABLE 5. The values [Ti+O.S(Nb+Ta)]/C of Nos. 21-28 range from 0.77 to 0.98.

TABLE 5 No. C Si Mn Ni Cr Ti Nb Ta T1+ %(Nb Ta) of Ti, Nb Ta and B causes the high temperature strength to improve greatly.

As indicated above, the ratio Ti 0.5(Nb +Ta)/C should be from about 0.25 to about 3, and preferably between about 0.6 and about 2.6. This critical relationship is revealed by data given below in TABLES 3 and 4, and illustrated in FIG. 2, with steels Nos. 1 1 through 19. It is to be noted that these steels do not contain 8; however, as demonstrated above, the addition of B to such steels increases the effectiveness of Ti and Nb. The components of such steels are set forth (in weight percent) in TABLE 3.

TABLE 6 KgJmmfl 9.0 9.2 11.0 13.0 13.6 11.2 10.5 7.7

TABLE 3 7 No C Si Mn Ni Cr T1 N1) Ta C 0. 07 0. 1.38 10. 03 18. 0'2 Tr. T1. 0

0. 0T 0. 17 1. 25 10. 27 18. 07 0. 02 0. 050 0. S0 0. l5 0. 55 1.51 0. 08 1T. 00 0. 0T 0. 223 1. 2 0.11 0. 51 l. 50 J. H 17. 10 0.10 0.10 1.1 0. 10 0.05 1.15 9. 0S 1?. 50 0. 011 0.18 l. 8 IT 0. 0 11.-10 1.19 10.-l. 18. 28 1.10 0. .31 2.0 l.\ 0. 0.\ 0.51 L-hi 10.17 18.10 0.15 0.31 4.2 1 0. .70 1.11- 10. 0S 1: .11 0. 2S 0. 10 ti. 0

Steels ll, 18 and 19 are included for comparative purposes, since the ratios fall outside the specified critical range. Steels 12 through 17 are illustrative of steels, except for their B content, come within the scope of this invention.

Steels ll-l9 were hot rolled, heated at 1100C. and

Data in TABLE 6 are also shown graphically in H0. 2. Such data demonstrate that substantially higher creep rupture values are realized with steels having a value of from about 0.15 to about 0.85 for Ti/- Ti+0.5(Nb+la).

Thus, according to the present invention, the insufficient heat-resisting strength of the steel is improved remarkably at lower cost since relatively expensive M0 is omitted.

The austenitic heat-resisting steels of the present inand having a ratio of Ti 0.5(Nb+fla)/C of from 0.25 vention may be employed broadly in a variety of industo 3, and a ratio of Ti/Ti 0.5(Nb+Ta) of from 0.15 trial circles. to 0.85.

What is claimed is:

- i 2. A steel of claim 1, wherein the ratio of Ti 1. An austenitlc heat resisting steel consisting essen 5 is from 0.6 to 2'6.

tially of, in weight percent,

3. A steel of claim 1, wherein the ratio of Ti/Ti g 32,2 0.5(Nb+Ta) is from 0.15 to 0.75. Mn up to 2 5.0 26 4. A steel of claim 1, wherein the Ti content 15 from Ni 7.0 m 22 0.02 to 0.10. Ti 0.00l to 0.30 g gjg g g fggg 5. A steel of claim 1, wherein the Nb+Ta content is Fe and unavoidable from 0.02 to 0.23. impurities balance, 1:

i;@EfiSflATESATENT' OFFICE QERWMTE @F QRR Q N Patent No. I 3.751; 244 I I, naeai August 1973 Invenfiofls) 'TOI-IRU MIMINo a: a1"

t "error appears in the above-identified patent It is certified. the I y corrected as shown below:

and that said Letters Patent are hex-e5 Abstract, line '5': replace" "0.05" with 00050 Column 3,

Table 3, No. 17: under "Ti", replace "1.10"

Column 4, 'Tebl'e 4-, line 7 under replace "10.0"

with --I-10;5---

Column 4, line 64: replace m. with ---Ta--- Column 5, line 10: are-space V"7.O to 22" to fall in correct-column beneath "15.0 to 26".

Signed and sealed this 1st day of October 1974.

(SEAL) Attest: V V

MCCOY M. GIBSON JR, 4 (I c. MARSHALL DANN A'ttest ing Officer Commissioner of Patents USCOMM-DC 6037 6-P69 FORM PO-105Q (10-69) {I U.S. GOVERNMENTP RINTING O FFICE: 1959 0 -3664! 

2. A steel of claim 1, wherein the ratio of Ti + 0.5(Nb+Ta)/C is from 0.6 to 2.6.
 3. A steel of claim 1, wherein the ratio of Ti/Ti + 0.5(Nb+Ta) is from 0.15 to 0.75.
 4. A steel of claim 1, wherein the Ti content is from 0.02 to 0.10.
 5. A steel of claim 1, wherein the Nb+Ta content is from 0.02 to 0.23. 